Field of the Invention
The present invention relates to the calcination of raw cement meal using oxy-combustion, the calcination of raw meal being an essential step in the production of cement clinker.
Related Art
The cement industry is an important emitter of the greenhouse gas CO2.
Within the cement production process, significant amounts of CO2 are more particularly generated during the decarbonation of raw meal (CaCO3) to lime (CaO) via the following reversible equilibrium reaction:
                                          CaCo            3                    ⁢                      ⟷            K                    ⁢          CaO                +                              CO            2                    ⁢                                          ⁢          Δ          ⁢                                          ⁢          H                    ≈              1800        ⁢                                  ⁢                  kJ          /          kg                ⁢                                  ⁢        K              =                  A        ⁢        exp            (                        -                      E            a                          RT            )        ,so that about 80% of the CO2 generated by a cement plant is produced at calciner level.
As explained in the article “The oxycombustion option” published in the May 2014 issue of the INTERNATIONAL CEMENT REVIEW 37, the cement industry has made considerable efforts to lower its CO2 emissions through the use of alternative fuels, lower specific heat consumption in kiln systems and a decrease of the clinker factor with the addition of supplementary cementitious materials leading to CO2 reduction by 20-30%.
A possible route for further CO2 mitigation lies in the application of carbon capture and storage technology (CCS) or carbon capture, storage and utilization technology (CCSU). This entails capturing CO2 from the cement plant's flue gases for storage or for use in other industrial applications.
The air used in conventional combustion processes consists mainly of nitrogen (about 78% vol), said nitrogen also forming the main constituent of the flue gas generated by air-combustion.
Several technologies have been developed to extract and capture CO2 from such flue gases, in particular for the power industry.
The current reference technology for capturing CO2 present in flue gases is amine scrubbing.
This process consists of extracting the CO2 fraction from a post-combustion flue gas by flushing the gases with an amine sorbent, regenerating the solvent by steam stripping, thus releasing nearly pure CO2, and recycling the stripped solvent to the absorber. Although this technology is very efficient, it is also quite expensive.
An alternative to post-combustion amine scrubbing is the use of oxycombustion.
In the oxycombustion process oxygen and recycled flue gas replace the conventional combustion air, so as to directly generate a CO2-rich flue gas during combustion and thereby to reduce downstream CO2 purification costs.
In a cement plant, oxycombustion can be applied either to the full production line (i.e. in both the calciner and rotary kiln section), such a process being referred to as “full oxy-firing”, or solely at the calciner stage, referred to as “partial oxy-firing”. In the comparative study “CO2 Capture in the cement Industry”, Report no 2008/3, published by the International Energy Agency (IEA), it was concluded that partial oxy-firing is the most cost-effective and lowest-risk configuration for retrofitting an existing cement plant. The IEA report also concluded that partial oxy-firing was cheaper than post-combustion amine scrubbing technology.
However, operating a calciner in oxycombustion mode has a major impact on the abovementioned decarbonation reaction because of the increase in CO2 partial pressure. Indeed, the desired decarbonation reaction only occurs if the equilibrium pressure—which strongly depends on the temperature—exceeds the surrounding CO2 partial pressure.
In order to counteract the high CO2 content linked to oxycombustion, it would therefore be necessary to operate the calciner at higher average temperature.
In calciners operating with air-combustion, the atmosphere contains typically from 25% vol to a maximum of 35% vol CO2. The corresponding equilibrium temperature of the decarbonation reaction is in the range of 800° C. to 850° C. According to the abovementioned study “CO2 Capture in the cement Industry”, the switch from air-combustion to oxy-combustion in a calciner would require a calciner temperature increase of around 80° C. to compensate for the increase in CO2 partial pressure.
As recognized in the article “The oxycombustion option”, operating the calciner at higher average temperatures entails an increased risk of hotspots within the calciner, even more so as burning fuel with oxygen is known to generate high-temperature product gases.
Such hotspots are responsible for disruptive material build-ups within the calciner leading to costly calciner shutdowns, the alternative being to operate the calciner at lower temperature, which would result in a significant deterioration of the process efficiency (lower calcination degree). This problem is even more manifest when fuels, such as petcoke, are used which require high temperatures and high residence times in order to achieve substantially complete combustion. Such fuels are frequently used in the cement industry in order to lower production costs.